Many medium head hydro schemes use HDPE (polyethylene) pipe which is jointed using butt fusion welding techniques. This welding process leaves a 'bead' - a squeezed out ring of plastic - both outside and inside the pipe at each joint. The beads on the inside of the pipe are difficult to remove but they cause an increase in flow friction, particularly at full flow rate. I once tried to calculate the effect and came out with something between 1% and 2% additional pressure loss. Because every scheme is different it is not possible to have a control experiment with one pipe de-beaded and and identical one not de-beaded to see if there is a real difference.

So here is a question - do you insist that the pipe is de-beaded? Or perhaps just over size the pipe? Or perhaps think that the difference is so small that it is not worth worrying about?

I suppose it is readily calculable and should be taken into account, but I have not tended to ask for debeading, just erred on oversizing the pipe on the basis that furring of the pipe over time probably might be more significant. It has often been enough of a task to get the contractor / plant operator to sufficiently raddle the backfill to avoid nasty sharp stony backfill. Our own schemes have generally been 100kW ish size so quite simple. What I have been agitated about before has been things like abrupt contractions when using SDR 26 say pipes, when getting an adaptor to bolted flange or similar, which is usually made in part from an SDR 11 or 16 wall thickness pipe, with a horrible sharp contraction internally where the two are joined by the pipe manufacturer.

The whole pipeline friction/velocity thing is an interesting dilemma - do you size for minimal friction at full flow, which risks dropout and sedimentation ,or size for self cleaning at higher flows which reduces head?It would be interesting to have your thoughts. What is reasonable pressure drop? 10%? And how about design velocities for self cleaning - is 2.5m/s high enough? My experience is that it's not.HuwHuw ScourfieldDwr Cymru Welsh Water

huw scourfield wrote:- do you size for minimal friction at full flow, which risks dropout and sedimentation ,or size for self cleaning at higher flows which reduces head?It would be interesting to have your thoughts. What is reasonable pressure drop? 10%? And how about design velocities for self cleaning - is 2.5m/s high enough? My experience is that it's not.r

Good to see the discussion starting.

My feeling is that Hydro should be designed for the maximum affordable efficiency at all times. Deliberately reducing efficiency so that the pipe is self cleaning is simply not right. As Huw says it is difficult to determine what volocity stops drop out of sediment - what would happen if you attempted to use a smaller pipe for this purpose and it did not 'self-clean'? You would end up with a lower efficiency system and still need to have the down time to clean.

On the de-beading discussion - When I first contemplated this I went to an engineering professor of mine - he made an attempt to calculate and came up with a loss of between 1% and 2% for the weld beads. One a typical 500kW run of river scheme that might be 15 to 30 MWh per year, over 50 years could be 1.5GWh. Not acceptable to me.

On penstock overall efficiency - I always aim for greater than 94% on average. This means that the efficiency is higher when the pipe is new and reduces as cleaning is required. On some of the recent schemes I have worked on this means starting with a 98% efficiency.

Experience in the wider water industry differs depending on waters. In West Wales upland reservoir sites are very prone to manganese/peaty deposits, which even at velocities of 2.5m/s tend to adhere to most surfaces. So yes, agree that in trying to maintain self cleansing you could be running at lower outputs and achieving nothing. I have an image of 30 years worth of build up on a 300mm main, but haven't worked out how to post it up..Huw